Oxidative Ionization Under Certain Negative-Ion Mass Spectrometric Conditions

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• 作者：Isra Hassan ; Julius Pavlov ; Ramu Errabelli…
• 关键词：Oxidative ionization ; HePI ; APCI ; ESI ; Hydroquinone ; Thermometer ions ; Ambient ionization
• 刊名：Journal of The American Society for Mass Spectrometry
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：28
• 期：2
• 页码：270-277
• 全文大小：
• 刊物主题：Analytical Chemistry; Biotechnology; Organic Chemistry; Proteomics; Bioinformatics;
• 出版者：Springer US
• ISSN：1879-1123
• 卷排序：28

文摘

1,4-Hydroquinone and several other phenolic compounds generate (M – 2) –• radical-anions, rather than deprotonated molecules, under certain negative-ion mass spectrometric conditions. In fact, spectra generated under helium-plasma ionization (HePI) conditions from 1,4-hydroquinone and 1,4-benzoquinone (by electron capture) were practically indistinguishable. Because this process involves a net loss of H• and H+, it can be termed oxidative ionization. The superoxide radical-anion (O₂–•), known to be present in many atmospheric-pressure plasma ion sources operated in the negative mode, plays a critical role in the oxidative ionization process. The presence of a small peak at m/z 142 in the spectrum of 1,4-hydroquinone, but not in that of 1,4-benzoquinone, indicated that the initial step in the oxidative ionization process is the formation of an O₂–• adduct. On the other hand, under bona fide electrospray ionization (ESI) conditions, 1,4-hydroquinone generates predominantly an (M – 1) – ion. It is known that at sufficiently high capillary voltages, corona discharges begin to occur even in an ESI source. At lower ESI capillary voltages, deprotonation predominates; as the capillary voltage is raised, the abundance of O₂–• present in the plasma increases, and the source in turn increasingly behaves as a composite ESI/APCI source. While maintaining post-ionization ion activation to a minimum (to prevent fragmentation), and monitoring the relative intensities of the m/z 109 (due to deprotonation) and 108 (oxidative ionization) peaks recorded from 1,4-hydroquinone, a semiquantitative estimation of the APCI contribution to the overall ion-generation process can be obtained.